Apparatus and methods for through substrate via test

ABSTRACT

A stack of vertically-connected, horizontally-oriented integrated circuits (ICs) may have electrical connections from the front side of one IC to the back side of another IC. Electrical signals may be transferred from the back side of one IC to the front side of the same IC by means of through substrate vias (TSVs), which may include through silicon vias. Electronic apparatus, systems, and methods may operate to test and/or replace defective TSVs. Additional apparatus, systems and methods are disclosed.

PRIORITY APPLICATION

This application is a divisional of U.S. application Ser. No.13/179,247, filed Jul. 8, 2011, which is a divisional of U.S.application Ser. No. 12/173,722, filed Jul. 15, 2008, now issued as U.S.Pat. No. 7,977,962, all of which are incorporated herein by reference intheir entirety.

BACKGROUND

The semiconductor device industry has a market-driven need to continueto reduce the size of semiconductor devices to obtain higher performanceand lower power consumption. To increase the amount of memory in alimited space, memory devices may be stacked in a vertical arrangement.Electrical connections through a memory device substrate may be providedby conductive vias, each at least partially through the substrate, fromone surface of the memory chip substrate to the other surface. Whensilicon technology is used, these may be referred to as through siliconvias. The ability to save space by stacking and directly connectingmemory chips with high reliability electrical connections becomes moredifficult as dimensions are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a vertical stack of electronic devices, according tovarious embodiments;

FIG. 2 is an electrical schematic of a device having the stackarrangement of FIG. 1, according to various embodiments;

FIG. 3 is an electrical schematic of a device having the stackarrangement of FIG. 1, according to various embodiments;

FIG. 4 is an electrical schematic of a device having the stackarrangement of FIG. 1, according to various embodiments;

FIG. 5 illustrates a method of testing TSVs, according to variousembodiments;

FIG. 6 illustrates a simplified block diagram of a system including acontroller coupled to an electronic device formed in accordance withvarious embodiments; and

FIG. 7 illustrates an electronic system having devices formed inaccordance with various embodiments.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific aspects and embodiments inwhich the present invention may be practiced. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the present invention. Other embodiments may be used andstructural, logical, and electrical changes may be made. The variousembodiments are not mutually exclusive, as some embodiments can becombined with one or more other embodiments to form new embodiments.

The term “substrate” as used in the following description may includeany structure having an exposed surface with which to form an integratedcircuit (IC) structure. The term “substrate” is understood to includesemiconductor wafers and is also used to refer to semiconductorstructures during processing and may include other layers that have beenfabricated thereupon. A “wafer” and a “substrate” each include doped andundoped semiconductors, epitaxial semiconductor layers supported by abase semiconductor or insulator, as well as other semiconductorstructures well known to one skilled in the art. The term “conductor” isunderstood to generally include n-type and p-type semiconductors anddoped regions in semiconductors. The term “insulator” or “dielectric” isdefined to include any material that is less electrically conductivethan the materials referred to as conductors or as semiconductors. Theterm crystalline is understood to not be limited to large singlecrystals having a specified crystallographic orientation, but mayinclude polycrystalline materials having a large number of moderatelysized crystals having various crystallographic orientations. The term“amorphous” is not limited to a solid material having a completelydisordered or glassy structure, but may include materials having somecrystalline order over short distances—on the order of ten atomicseparations or less.

The term “horizontal” is defined as a plane parallel to the conventionalplane, i.e., the top surface of a substrate, regardless of theorientation of the substrate. The term “vertical” refers to a directionperpendicular to the horizontal. Prepositions, such as “front”, “back”,“on,” “side” (as in “sidewall”), “higher,” “lower,” “over,” and “under”are defined with respect to the conventional plane, i.e., the topsurface of the substrate, regardless of the orientation of thesubstrate. The following detailed description is, therefore, not to betaken in a limiting sense.

Vertically stacked devices, such as vertically stacked ICs, having anelectrical input or output (i.e., I/O) connection between the top of oneIC to the bottom of another IC disposed directly above the first IC maybenefit from shortened IC to IC signal path lengths. A typical IC mayhave active devices, such as transistors only on the top surface of theIC, and may use a through silicon via to transfer the electrical signalfrom a bottom contact to the top of the IC. An IC may include asubstrate material that is different from silicon. A conductive via froma bottom to a top of a substrate, or at least partially through asubstrate, may be termed a though substrate via (TSV). Thus, “TSV” isused herein to mean a through substrate via. A through silicon via, whensilicon is used in a substrate, is an embodiment of a TSV. Theelectrical quality of the TSV used in a particular assembly may bedetermined by measuring the impedance, e.g., either or both of theresistance and the capacitance, of the stack including TSVs in each ofthe ICs.

FIG. 1 illustrates a vertical stack 100 of electronic devices, having alogic chip 102 serving as a base device. The described arrangement mayalso have a memory chip, a microprocessor, or any other type of IC chipas the base device without affecting the principles of the embodiment.

To reduce the amount of power consumed, and the access time involvedwhen the logic chip 102 accesses memory (106, 112, 118, 124), theelectrical path between the logic chip 102 and the memory should be asshort as possible, and have a low resistance. One method of obtainingshort electrical paths includes the use of conductive bumps 104,typically formed of gold, copper, or solder, on I/O pads of the chip102. These bumps 104 may be directly connected to backside contacts onthe memory chip 106. It should be noted that the chip 106 may also beanother logic chip or any other IC chip without loss of generality withrespect to the described embodiment.

To convey electrical signals from chip 102 and the bump 104 to thecircuitry on the top surface of the memory chip 106 with the shortestpath length, it may be convenient to use a TSV 108. Additional memorychips may be vertically arranged above the memory chip 106 to addressthe same I/O connections included in the memory chip 106. To accomplishthis, the TSV 108 may connect to both the circuitry of memory chip 106and to a conductive bump 110, providing access to a second memory chip112. TSV 114 connects to the circuitry of chip 112, and to a conductivebump 116 providing signal access to memory chip 118. Chip 118 is shownwith a TSV 120 that connects to the circuitry of memory chip 106 and toconductive bumps 122.

Bumps 122 connect to the bottom surface of memory chip 124 and TSV 126connects the chip 118 to the circuitry of the chip 124. Four memorychips 106, 112, 118, and 124 are shown in the figure, but there is nogeneral limit as to how many IC chips of any type may be used in thearrangement shown to provide short electrical paths between variousstacked chips.

FIG. 2 is an electrical schematic of a device 200 having the stackarrangement of FIG. 1, according to various embodiments. The dottedboxes 102, 106, 112, 118, and 124 indicate that the circuitry containedwithin the dotted box is from the same numbered feature of FIG. 1, withthe illustrative logic chip 102 being shown as the base device of thevertical stack and memory chips 106, 112, 118, and 124 sequentiallylocated above the logic chip 102. It is not necessary that a logic chipbe the base device, and any order, or different types of ICs may be usedin the arrangement shown.

Due to randomly distributed manufacturing defects, certain TSVs (e.g.,TSV 216) may be totally open or short-circuited, or may haveunacceptably high resistance and/or capacitance, and thus may add tochip communication time (e.g., by way of an added RC delay). Forexample, a TSV having higher than normal resistance may add 100picoseconds signal delay. Therefore, measurement of the TSV quality inassembled chip stacks may be beneficial if the complexity of addedcircuitry to perform the measurement and its associated semiconductorarea penalty are low.

In an embodiment, the TSV quality of stack 100 may be determined. Oneinput of a sense amp 218 may be connected to a reference voltage (Vref)202, and a second sense amp input may be connected to a current source222 and to the TSV stack, which may comprise one or more seriesconnected TSVs. The voltage (V1) at the second input of sense amp 218 isa measure of the total TSV stack resistance (Rx). In an embodiment, thememory chips 106, 112, and 118 are not powered up during the measurementof TSV quality. In an embodiment, the memory chip 124 has a selectedtermination resistance (Rterm) 224 to a selected termination voltage(Vterm) 204. The Rterm value may be selected to be small compared to thetypical resistance of the TSV stack, for example Rterm may beapproximately 50 ohms, while Vterm may be approximately 0.6 volts toimprove sensitivity.

In an embodiment, the current source 222 of logic chip 102 may includeadditional power drivers to provide a range of test currents, forexample 0.25 to 2.5 milliamps, which may comprise direct current. In anembodiment, the individual memory chips 106, 112, 118, and 124 may haveindividual switches connecting or disconnecting an individual TSV to thechip circuitry for a TSV test. Various combinations of chips may beturned on or off during different parts of a test. The individualswitches may be realized in various formats, for example by use oftransistors 208, 210, 212, and 214. In an embodiment, the current source222 and the sense amp 218 are located as extra circuitry in the basedevice, such as the logic chip 102, where the base device may be a chiphaving other functions, for example a memory chip or a microprocessor.

The resistance of a TSV stack may be determined by sweeping a range ofcurrent values at the output of current source 222, and determining trippoints for a high and a low Rx value at sense amp 218 for the connectedTSV stack. By driving stepped currents from 0.25 to 2.5 milliamps in0.25 milliamp steps, the voltage measured at 218 may differ by more than0.10 volts over a TSV stack resistance range of 5 ohms to 50 ohms, thusthe value of the voltage at 218 provides a measurement of TSV quality.

Reference voltage 202 and supply voltage 204 may connect to a series ofsense amps such as 218 and 220. Reference voltage 202 may be provided byone or more power supplies, or any stabilized voltage supply between thepower supplies, and supply voltage 204 may be provided by a groundconnection. There may be sense amps attached to each vertical stack ofTSVs that are to be tested, which may be more than the two shown,including any number up to the total number of I/O pads on the memorychips or other type of IC chips to be tested. The reference voltage(Vref) may be varied to step through a series of TSV tests to determinethe quality of partially open TSVs by measuring the voltage (V1) at thesecond input of sense amp 218 as a measure of the total TSV stackresistance (Rx).

In an embodiment, the sense amp 218 may have additional control inputsto form a control circuit 206. The TSVs may be tested by comparing ameasured RC delay in a ring oscillator signal to a signal from aprogrammable delay line. The amount of delay may be incremented until aparticular number of good TSV stacks are found. If a defective (e.g.,poorly performing stacks having a resistance greater than a selectedvalue) TSV stack is found, in an embodiment, the circuit may comprise aselected number of redundant TSV stacks and circuitry to replace adefective TSV stack with up to a selected number of tested goodredundant TSV stacks.

In an embodiment, the control circuit 206 provides a reference clockedge to a selected one of the memory chips, for example 124, which isselected by activation of transistor 214 initiating the drivers on thememory 124. The output of the TSV stack at the sense amp in controlcircuit 206 is latched for each increment in the delay line value andanalyzed to determine the TSV quality, such as RC delay time, and tocompare the TSV to other TSV stacks on the selected memory device. If aTSV is close to a selected resistance limit, it may be replaced by oneof the redundant TSV stacks. This may continue until the best TSV stacksare found.

In various embodiments, the test may be done on operational devices asthey age to evaluate device degradation and potential failure rates.Various embodiments use the resistance of the TSV stack and the RC delayto determine a quality value, enable selection of the TSV stacks in agroup of main and redundant stacks considered to be the best in thegroup, and to permit repair and replacement of defective TSV stacks.

FIG. 3 is an electrical schematic of a device 300 having the stackedarrangement of FIG. 1, according to various embodiments. Device 300includes components that determine a TSV stack's quality by measuringthe resistance of the TSV stack and testing the TSV stacks to select thebest ones of the total main TSVs and redundant TSVs. A direct currentmethod may be used for measurement and testing that does not include ahigh-speed clock. The sense amps used in the method may be the existingsense amps on the memory chips 106, 112, 118 and 124 used in normaloperation and function of these chips and thus may use less additionalcircuit space as compared to various embodiments that use sense amps onthe base chip as discussed with respect to FIG. 2. As in the previousfigure, the dotted boxes are numbered as in FIG. 1 to show in which chip(i.e., 102, 106, 112, 118, or 124) the disclosed circuitry is contained.

In the previously discussed method, the RC delay measurement was madedynamically and used a termination resistance and voltage in conjunctionwith ring oscillator edge delays. Device 300 includes a voltage supply302, such as Vcc, connected via an enable transistor 306 to the selectedTSV stack, such as the stack of TSVs including TSV 320. The top memory124 in the stack includes connections from the three TSV stacks shown inthe figure to respective ones of sense amps 308, 310, or 312. It shouldbe noted that the present embodiment is not limited to four memory chipslocated on a logic chip as shown; any number or type of IC chip may beused. The sense amps may be selected from among those normally used inthe operation of a memory device if the top chip is a memory or containsembedded memory elements and thus not add substantial additional circuitarea. In various embodiments, memory chips 106, 112, and 118 may havesimilar sense amp circuitry to that shown for the TSVs in chip 124.

The connection between the TSV stack to be tested and the sense amp mayinclude circuitry 314, 316, or 318, comprising a current source and aground reference. The current source may provide approximately 200 microamps. The other input to the sense amps 308, 310, and 312 may be drivenby a voltage reference (Vref) 304, which may be set by a control circuit(not shown for simplicity). The voltage reference 304 may besequentially stepped up or down in value in comparing adjacent TSVstacks.

The output of the sense amps 308, 310, and 312 may go to a latchcontrolled by the clock for testing. The enable transistors 306 may beconstructed to have essentially the same “on” resistance to the voltagesupply 302 and to have a low capacitance and “off” state leakage. Device300 may test TSVs by comparing the measured resistance values as theVref value is stepped through a selected range by a control circuit. Therelative resistance of the TSV stacks may be compared to select the bestTSV stacks without a determination as to whether or not any particularTSV is in a specified range. It should be noted that while only threeparallel TSV stacks are shown in the described embodiment, there is nogeneral limit to the number of TSV stacks that may be compared. The TSVstack test includes adjusting a reference voltage connected to a senseamplifier to provide a plurality of voltage steps of a selected value,and then determining when the reference voltage equals a voltage on theother input of the sense amplifier. The sense amplifier may be connectedto an electrical path formed of a series of connected vias and driven bya current supply. The reference voltage values for each of theelectrical paths are compared and the quality of each electrical pathcan be tested in this manner. This may also be done as part of areliability test, an infancy failure test or as a lifetime test.

FIG. 4 is an electrical schematic of a device 400 having the stackarrangement of FIG. 1, according to various embodiments. A device 400includes a lower voltage supply 402 positioned between the voltagesupply 428 and an upper voltage reference Vref 404. Lower voltage supply402 is separated from voltage supply 428 by a first resistance R1 426.Voltage supply 402 is separated from the upper reference supply 404 bythe on resistance Rsw of an enable transistor 406, a second resistanceR2 424, and the value of the total resistance of TSV 420. Since TSVstack 420 is used to bias upper reference voltage 404, it is useful toverify that TSV 420 is a known good stack in this embodiment. Thearrangement shown in device 400 may allow testing of the three shown TSVstacks using direct current and without need for a clock, and maydetermine an absolute resistance measurement of the total resistance ofeach of the TSV stacks. Because the upper reference is biased by thecurrent flowing through TSV stack 420, this method benefits from TSVsthat provide low voltage drop. It is not necessary that the TSVresistance be low, since the amount of current passing through the TSVstack is low, or may be considered to be essentially zero. However, theTSV stack should conduct a measurable amount of electrical current—onthe order of 100 microamps given a supply voltage Vref of 3.3 volts.

Each TSV stack to be measured has a sense amp (408 and 412), which maybe selected from the sense amps used in normal operation in a memorychip such as 124, or there may be a sense amp formed for this purpose oneach chip of the stack 102, 106, 112, 118, 124, or any combination ofthese. Each sense amp 408 and 412 may have a current source with groundconnection 414 and 418 respectively. A current source 422 may beconnected to both the resistance R2 and the TSV stack 420 to provide areference current Iref. The value of Iref is chosen based upon thevalues of TSVs to be measured (for example 420), R2, and thesensitivities of the sense amps 408 and 412, and may be about 1milliamp. The sense amps 408 and 412 may be used to determine if theircorresponding TSV resistance is greater or less than R2 by comparing theupper (Vref) and lower (Vtsv) sense amp inputs, and the value may belatched for use in repair and redundancy decisions.

The upper reference Vref 404 in this embodiment may have a valuedetermined by the formula Vref=Vcc−Iref(R1+R2+Rsw). The voltage of theTSV stack Vtsv is determined by the formula Vtsv=Vcc−Iref(R1+Rtsv+Rsw).Vtsv may comprise the voltage value at the TSV input of the sense amps408 and 412, while Vref may comprise the voltage value at the otherinput of the sense amp. The high gain sense amp 408 then amplifies thedifference between Vref and Vtsv and provides a reliable comparisonmeasurement.

The resistance of the TSV stack Rtsv may be determined from thecombination of the above formula for Vref and Vtsv by use of the formulaVtsv−Vref=Iref(R2−Rtsv). Thus the difference output of the sense amp408, plus the known value for the reference resistance 424 R2, and thevalue for the current source 422 Iref, provide a method of calculatingthe TSV stack resistance Rtsv. The value of the reference current supply422 Iref may be varied to provide further testing capability, such asdetermining lifetime degradation and sensitivity.

FIG. 5 illustrates a method of testing TSVs, in accordance with variousembodiments. The method of testing TSVs includes selecting TSV stacksfor testing at step 502 and providing a termination resistor and atermination voltage level at step 504. Connecting a reference voltagesource, a current source and a voltage measuring device to the TSV atstep 506 enables the application of a current across both of the TSVstack to be tested and the termination resistance at step 508, whichresults in an IR voltage drop. Measuring the voltage drop at step 510provides a relative estimation of the resistance of the TSV stack atstep 512. Repeating the test method for another TSV stack at step 514permits the TSV stacks to be compared at step 516, which enables theselection of the best TSV stacks for use in a functional device.

FIG. 6 illustrates a simplified block diagram of a system 500 includinga controller coupled to an electronic device formed in accordance withvarious embodiments. The system 600 includes one or more devices,including at least one device having stacked TSVs tested, selected, orrepaired according to various disclosed embodiments. The electronicsystem 600 may include a controller 602, a bus 604, and an electronicdevice 606, where bus 604 provides electrical conductivity betweencontroller 602 and electronic device 606. In various embodiments, thecontroller 602 and/or electronic device 606 may include a device havingstacked TSVs tested, selected, or repaired as previously discussedherein. Electronic system 600 may include information handling devices,wireless devices, telecommunications devices, fiber optics devices,automotive devices, electro-optic devices, mobile electronics, handhelddevices, and computer systems. Electronic device 606 may comprise amicroprocessor, a floating point unit, an arithmetic logic unit, amemory device, a multiplexer, an address decoder, a power controller, orany other electronic device used in computers, telecommunications,sensors, displays, and other products.

FIG. 7 depicts a diagram of an electronic system 700 having at least onedevice formed in accordance to the various embodiments, including acontroller 702 and a memory 706. Controller 702 and/or memory 706 mayinclude at least one device having stacked TSVs tested, selected, orrepaired as discussed herein.

The system 700 may also include an electronic apparatus 708 and a bus704, where the bus 704 may provide electrical conductivity and datatransmission between controller 702 and electronic apparatus 708, andbetween controller 702 and memory 706. The bus 704 may include anaddress, a data bus, and a control bus, each independently configured.The bus 704 may use common conductive lines for providing address, data,and/or control, the use of which may be regulated by the controller 702.

In some embodiments, the electronic apparatus 708 may include additionalmemory devices configured similar to the memory 706 with devices havingstacked TSVs tested, selected or repaired, as disclosed herein. Someembodiments may include an additional peripheral device 710 coupled tothe bus 704. In an embodiment, the controller 702 comprises a processor.Any of the controller 702, the memory 706, the bus 704, the electronicapparatus 708, and peripheral devices 710 may include at least onedevice having stacked TSVs tested, selected, or repaired in accordancewith the disclosed embodiments.

System 700 may include, but is not limited to, information handlingdevices, telecommunication systems, mobile electronic devices such aslaptop computers, handheld personal electronic devices such as personaldigital assistants (PDAs) and palm tops, handheld communication devicessuch as cell phones, digital cameras and DVD recorders, and computers.Peripheral devices 710 may include displays, additional storage memory,or other control devices that may operate in conjunction with controller702 and/or memory 706.

Various embodiments may include the activities of determining anelectrical quality value for a stack of vertically connectedhorizontally oriented ICs having electrical connections from the frontside of one IC to the back side of another IC. The electrical signalsmay be transferred from the back side of one IC to the front side of thesame IC by means of a TSV. The testing, selection of good TSVs, andreplacement of defective TSVs with redundant TSVs may be accomplished invarious ways similar to the three embodiments disclosed herein. Variousembodiments measure the absolute value of resistance through the TSVusing two additional resistors and sense amps from a memory device,which is affected by defects in TSVs used to provide bias transfer tothe sense amp inputs. Various embodiment compare relative resistance ofTSVs in parallel using stepped voltage reference values, a clock, and aclocked latching scheme, which is relatively unaffected by variabilityin TSV resistance values. Such embodiments may be implemented usingminimal additional circuitry in the chips in which the TSVs aredisposed. RC delay value for TSVs may be determined by comparing thedelay of a ring oscillator signal to a variable delay line, using a ringoscillator, a variable delay line, a high speed clock, sense amplifiers,and a controller circuit added to a base logic chip. Thus, manyembodiments may be realized.

For example, in various embodiments, a method of testing a stack ofconnected electronic devices may include providing series connectedelectrical paths between selected I/O pad connections on each one of aplurality of electronic devices, and providing termination resistancesand termination voltages for the series connected electrical paths.Using a reference voltage, a current source, and a voltage measuringdevice allows test mode circuitry in each electronic device to determinean overall resistance value for each series connected electrical path bycomparing an IR drop caused by an imposition of current from the currentsource across the termination resistor and a sum of resistances formedby the series connected electrical path. In various embodiments, thetype of devices in the stack may include logic chips, state machines,microprocessors, and memory chips. In various embodiments, the voltagemeasuring device may be an added portion of a logic chip, which may forma base for mounting the remaining chips. In various embodiments, thetermination resistor and termination voltage may be provided in a memorydevice in the stack of connected electronic devices, and the overallresistance value may be determined by setting the current source tosweep a selected range of about 250 microamps to 2.5 milliamps and thendetermining a high trip point value and a low trip point value in a plotof the readout of the voltage measuring device. The current source mayprovide a DC current generated in one of the plurality of electronicdevices, and the high and low trip point may have as little as about a50 millivolt difference or as much as about a 500 millivolt difference,and may have a difference of approximately 10% between the high and thelow trip points. The I/O pad connection on each one of the electronicdevices may comprise a bi-directional I/O pad on a logic device or amemory device. In various embodiments, a stacked arrangement of ICs mayinclude a ring oscillator circuit forming part of one of the circuitsthat provides a signal to the series connected electrical path, and avariable delay line that compares the rate of an additional RC delaybetween the series connected electrical paths to determine relativequality factors for each series connected electrical paths.

In various embodiments a method may include comparing electricalresistances of electrical paths comprising a series connected pluralityof TSVs by adjusting a reference voltage connected to a first inputterminal of a sense amplifier circuit through a plurality of voltagesteps. This activity may be managed by an included controller circuit.The method may include connecting a second input terminal of the senseamplifier circuit to at least one of the electrical paths, connecting afirst current supply to one of the electrical paths, determining whenthe reference voltage equals the voltage on the second input terminal ofthe sense amplifier, and recording the reference voltage value for eachelectrical path. In this fashion, a ranked quality factor for each ofthe electrical paths may be obtained. In various embodiments, a secondcurrent supply may be connected to the second input terminal of thesense amplifier, which may be set to about 0.20 milliamps. In anembodiment, the sense amplifier may comprise an operational element of amemory circuit such as a memory circuit or chip, which is included inthe plurality of circuits forming the series connected plurality ofTSVs.

In various embodiments, a method of measuring a resistance value in eachone of a plurality of electrical signals may include connecting aplurality of integrated circuits to form a plurality of series connectedelectrical TSV stacks, connecting a first reference supply to a powersupply through a first resistor, connecting one end of a test seriesconnected electrical TSV string to the first reference supply through asecond resistor, forming a voltage divider. In an embodiment, a testenable transistor may be used to turn the testing device on and off. Thetesting device may be used as part of the end of fabrication functiontesting, and it may be used as a lifetime degradation test.

The resistance of each series connected TSV stack may be determined bymeasuring the output of a connected sense amplifier. Connect a first endof each TSV stack through an enable transistor to the first referencepower supply, and connect the other end of the TSV stack to a secondreference line. Connect each TSV stack to a first input of a senseamplifier, and connect the second input of the sense amplifier to thesecond reference line,. In an embodiment, the second reference line isnot directly connected to a power supply and is only biased by the testvia string. The sense amplifier may be part of a memory circuit in anintegrated circuit, and each one of the enable transistors may bestructured to have the same size, width, and electrical properties toprovide improved accuracy.

In various embodiments a method of testing a stack of interconnectedelectronic devices includes comparing the voltage drop caused by acurrent source across a termination resistor and the sum of a resistanceformed by a series connected electrical path between input/outputconnections on one electronic devices and another to determine thequality of the series connected electrical path.

In various embodiments, a circuit for measuring a resistance value inTSV stacks may include a floating potential reference, and a pluralityof operational amplifiers, for example the sense amplifiers found inmemory devices. Each operational amplifier may be connected between thefloating potential reference and a top portion of one of the TSV stacks.Enable switches may be connected between a bottom portion of the TSVstacks and a second potential reference. A combination of a firstresistor and a second resistor may allow the TSV stacks to be measuredand compared. The first resistor may be connected between the secondpotential reference and a power supply. The second resistor may beconnected between both the second potential reference and a bottomportion of a TSV stack, and between a top portion of the test seriesconnected electrical TSVs and the floating potential reference line. Inan embodiment, the test TSVs may be made as identical to each one of theplurality of TSVs to be measured as possible, which may be accomplishedby forming the test and vias on the same ICs at the same time ofmanufacture. A current source may be connected to the top portion of oneof the plurality of series connected electrical TSVs and one of theplurality of operational amplifiers to provide improved sensitivity.

In various embodiments, a circuit for measuring a resistance value inTSV stacks may include a first voltage reference source, and a pluralityof operational amplifiers, for example the sense amplifiers found inmemory devices. The plurality of operational amplifiers may be connectedto the first voltage reference source and connected to an end of one ofa plurality of series connected electrical through substrate vias. Thesecond end of the series connected electrical through substrate vias maybe connected to a second voltage reference source. The first voltagereference source may be a variable voltage to adjust the sensitivity ofthe resistance test, and may be varied during testing by the use of acontrol circuit. The circuit may further include current sourcesconnected to both the operation amplifiers and the series connectedelectrical through substrate vias to increase the ability to measuresmall resistance values. The circuit may include enable transistorsconnected between the series connected electrical through substrate viasand the second voltage reference source to turn the individual vias onand off for testing.

In various embodiments an apparatus may include a first potentialreference having a connected operational amplifier. The operationalamplifier may be a sense amplifier such as found in memory devices andlogic device circuits. The operational amplifier may also be connectedto a current source, and an end of series connected electrical throughsubstrate vias. There may also be a state machine, for example a microprocessor, a ring oscillator, or a variable delay line, connected to thefirst potential reference and an end of a series connected group of testthrough substrate vias. Each of the test through substrate vias may beconnected to an enable transistor and the enable transistor may beconnected to a selected portion of circuitry having a selectedresistance and capacitance. The selected circuitry may be the entiretyof a integrated circuit containing the through substrate via, and serveas part of an RC delay test. A second potential reference may beconnected to a second end of the series connected electrical throughsubstrate vias as well as to a termination resistor connected to an endof the series connected group of test through substrate vias. Theapparatus may further include a variable first voltage reference sourceenabled to vary during testing or during operation. The variation may beperformed by a control circuit to vary the first voltage referencesource in a preselected pattern or in response to test conditions andresults.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of embodiments of thepresent invention. It is to be understood that the above description isintended to be illustrative, and not restrictive, and that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Combinations of the above embodimentsand other embodiments will be apparent to those of skill in the art uponstudying the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a few embodiments for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted to require morefeatures than are expressly recited in each claim. Rather, inventivesubject matter may be found in less than all features of a singledisclosed embodiment. Thus the following claims are hereby incorporatedinto the Detailed Description, with each claim standing on its own as aseparate embodiment.

What is claimed is:
 1. A method of comparing electrical resistances,comprising: connecting an output of an adjustable reference to a firstinput terminal of a sense amplifier circuit; adjusting a level of theoutput by a plurality of steps of a selected size; connecting a secondinput terminal of the sense amplifier circuit to at least one of aplurality of electrical paths each comprising a series connectedplurality of vias; applying a first current having a selected value toat least one of the plurality of electrical paths; determining when theoutput equals a voltage on the second input terminal of the senseamplifier; recording the level for each of the plurality of electricalpaths; and determining a quality factor proportional to the voltage onthe second input terminal for each of the plurality of electrical paths.2. The method of claim 1, further comprising: connecting a secondcurrent supply to the second input terminal of the sense amplifier. 3.The method of claim 2, further comprising: providing the second currentsupply as a supply with a controllable variable output value.
 4. Themethod of claim 1, further comprising: adjusting the level with acontrol circuit.
 5. The method of claim 1, wherein adjusting the levelof the output by a plurality of steps of selected size comprisesadjusting a current level from 0.25 to 2.5 milliamps.
 6. The method ofclaim 5, wherein the steps comprise 0.25 milliamp steps.
 7. The methodof claim 1, further comprising ranking the quality factor for each ofthe plurality of electrical paths.
 8. The method of claim 7, whereindetermining the quality factor for each of the plurality of electricalpaths comprises determining the quality factor relative to otherelectrical paths of the plurality of electrical paths.
 9. A method,comprising: connecting a plurality of integrated circuits to form aplurality of series connected electrical through substrate via stacks;connecting a first reference to a power supply through a first resistorhaving a selected resistance value; connecting a first end of a seriesconnected electrical through substrate via stack to the first referenceby a second selected resistance coupled to a first transistor having atransistor resistance value; connecting a second end of the seriesconnected electrical through substrate via stack to a second reference;connecting a first end of each one of the series connected electricalthrough substrate via stacks through a second transistor to the firstreference; connecting a second end of each one of the series connectedelectrical through substrate via stacks to be measured through a firstinput of a sense amplifier; connecting a second input of each senseamplifier to the second reference; and determining a resistance of eachone of the plurality of series connected electrical through silicon viastacks by measuring an output value of each sense amplifier.
 10. Themethod of claim 9, where the second reference is not directly connectedto a power supply.
 11. The method of claim 10, wherein the secondreference is connected to the power supply by a through silicon viastring.
 12. The method of claim 9, where each sense amplifier is aselected sense amplifier in at least one of the plurality of integratedcircuits.
 13. The method of claim 9, wherein determining the resistanceof each one of the plurality of series connected electrical throughsilicon via stacks comprises: repeating the determining after theplurality of integrated circuits has been operated for a selected timeperiod in at least one of a normal operational mode, an acceleratedoperational mode, a static voltage stress mode, or an environmentalstress mode to evaluate through silicon via string degradation andfailure rates.
 14. The method of claim 9, wherein each sense amplifieris part of a memory circuit.
 15. The method of claim 9, furthercomprising turning on individual vias for testing.
 16. The method ofclaim 15, further comprising turning on the individual vias for testingby enabling via stacks with an enable transistor.
 17. The method ofclaim 16, further comprising connecting the first end of the seriesconnected electrical through substrate via stack to the first referencethrough the enable transistor.
 18. The method of claim 9, whereinconnecting the first reference to the power supply through the firstresistor having the selected resistance value comprises connecting aring oscillator through the first resistor.